Heat-shrinkable thermoplastics are known to be useful as flexible packaging materials for vacuum packaging various foodstuffs, including meat. Such plastic materials, however, while generally suitable for packaging meat, understandably have difficulties in successfully packaging sharp or bony products. For example, attempts to package bone-in primal cuts of meat usually result in an unsatisfactorily large number of bag failures due to exposed bone puncturing the bag.
The use of cushioning materials such as paper, paper laminates, wax impregnated cloth, and various types of plastic inserts have proved to be less than totally satisfactory in solving the problem, as they require large expenditure of materials and labor, and are subject to shifting off of protruding bones. The preparation of special cuts of meat or close bone trim with removal of protruding bones has also been attempted. However, this is at best only a limited solution to the problem since it does not offer the positive protection necessary for a wide variety of commercial bone-in types of meat. Furthermore, removal of the bone is a relatively expensive and time-consuming procedure.
Some time ago, the use of a bag having a patch thereon, i.e., a "patch bag", became a commercially-preferred manner of packaging a number of bone-in meat products. One of the first commercially-utilized patch bags utilized a heat shrinkable bag and a patch composed of two laminated VALERON.TM. high density polyethylene ("HDPE") films, each film having been highly oriented in the machine direction. In the laminated patch, the machine direction of a first HDPE lamina was oriented about 90 degrees with respect to the machine direction of a second patch lamina.
The VALERON.TM. HDPE patch performed well in preventing punctures from exposed bone. However, upon packaging a meat product in a heat-shrinkable bag having such a patch thereon, the corners of the patch delaminated from the bag upon shrinkage of the bag, due to the fact that the patch would not shrink as the bag shrunk. Customers perceived the delamination of the patch corners from the bag to be highly undesirable. Furthermore, the highly oriented HDPE films were opaque white due to the formation of voids during the orientation process.
Thus, the need arose for a patch which would provide the patch bag with a desired level of puncture-resistance, while at the same time being heat-shrinkable so that there would be no substantial delamination at the corners of the patch. Furthermore, although for some uses it was desirable to use an opaque patch, for other uses it was desirable to provide a substantially translucent or transparent patch.
Although ethylene/vinyl acetate copolymer ("EVA") was known to have the desired heat shrink properties for use in patches, it was discovered to lack the desired level of puncture-resistance obtainable using the VALERON.TM. HDPE patch. That is, EVA patches had to be much thicker than an HDPE patch in order to provide the same level of puncture-resistance. Furthermore, in addition to lacking the desired puncture-resistant character, EVA lacked abrasion-resistance, further diminishing its utility as the bulk polymer in the patch.
Surprisingly, linear low density polyethylene (LLDPE) was found to provide the combination of puncture-resistance, heat-shrinkability, abrasion-resistance, and even transparency, desired for use in a patch for patch bags. Within the last 10 years, patch bags having patches composed of LLDPE have come into widespread commercial use in the United States.
However, LLDPE has several drawbacks. For example, LLDPE is not easily processable as it causes high extruder back pressure if extrusion is attempted at relatively high speeds. Furthermore, because of its stiffness, LLDPE is difficult to orient, which necessitates that another polymer be blended with the LLDPE in order to permit the desired orientation of the LLDPE to provide a shrinkable patch. Furthermore, LLDPE will not heat-seal to itself, necessitating the use of another type of polymer if the patch is to be formed from a collapsed film tube, as is one of the most desirable processes for manufacturing patches.
Thus, it would be desirable to locate another polymer which can provide the combination of puncture-resistance and heat-shrinkability. Furthermore, it would be desirable that this other polymer also be capable of being manufactured as a substantially transparent film. Furthermore, it would be desirable if this other polymer was easier to extrude than LLDPE, had a stiffness low enough to avoid the need to blend a stiffness-reducing polymer therewith, and had the ability to be heat-sealed to itself.